Celestial tracking apparatuses are devices that track or face a given object in the celestial hemisphere during normal operations. Such apparatuses are typically configured for use as telescopes, radio telescopes, radar systems, directional antennas, solar collectors, solar dishes, and so forth.
Such tracking systems typically employ two independent systems to tilt the object, i.e., the telescope, radar dish, antenna, collector, and so forth, about two axes. In one arrangement, one axis is horizontal, commonly called “elevation”. The elevation axis is the axis about which the object may be tilted within an angular range of about ninety degrees, from the horizon to looking straight up. The other axis is vertical, commonly called “azimuth.” The azimuth axis is the axis about which the object may be rotated around the horizon. When a tracking system is configured for use as a solar collector, the azimuth positioning system may orient the solar collector approximately eastward at the beginning of the day and end the day with the solar collector oriented approximately westward. The angular rotation, i.e., the elevation range, depends upon the earth's latitude at which the solar collector is installed and the time of year. This exemplary arrangement enables positioning in three dimensions. Those skilled in the art will readily recognize that other arrangements of tracking systems exist for positioning in two and/or three dimensions. Those skilled in the art will readily recognize that other arrangements of tracking systems exist for positioning in two and/or three dimensions.
A number of tracking and control systems are available for positioning small, lightweight objects, and many of such tracking and control systems can achieve respectable tracking precision. Such small positioning systems typically use electrical motor-driven gear reduction drives, harmonic drives, and the like. However, in the field of solar-collecting arrays, economics is encouraging the use of larger and larger solar-concentrating arrays which require highly precise tracking of a very large object. Unfortunately, the highly precise tracking and control systems that are desirable for use with smaller objects are prohibitively expensive for larger systems.
Moreover, some prior art positioning systems for large objects, such as solar collectors, suffer from positional uncertainty (e.g., approximately 0.5° to 2°). Several factors are believed to contribute to this uncertainty. One factor is the use of bearings and other linkages to couple between a driving mechanism and the driven mechanism. The bearings and other linkages can introduce some degree of backlash and non-linearity that detract from the precision of the positional movements. In addition, loads can deflect and deform the drive mechanism also detracting from the precision of the position movements, unless the system is stiff.
Large systems favor hydraulic drive mechanisms over electrical drive mechanisms for reasons of cost and strength. However, another factor that contributes to positional uncertainty in hydraulic systems is attributed to the compressibility of the oil used for the hydraulic systems. Typical hydraulic oils may compress roughly 0.5% for each 1000 psi. The pressure is dependent upon the load, and the load, in turn, may vary depending upon wind conditions and orientation of the load relative to gravity. In one exemplary system, the hydraulic system for a solar collector has a stroke of approximately 24 inches. Accordingly this 0.5% per 1000 psi uncertainty can translate into considerable uncertainty in position. Further uncertainty may result to the extent that air is entrained in the oil. In addition, the steel rods and other structural members can compress by varying amounts depending upon the loads experienced. For a longer member, such as the hydraulic system having a 24 inch stroke, the compressibility of the steel rods and other structural members will translate into a greater amount of position uncertainty.
Tracking assemblies that present large surface areas to the wind are subject to considerable wind stresses. When the wind exceeds a given speed, these stresses can become destructive. Accordingly, such tracking systems desirably require that the system be capable of going into a wind stow position to avoid damage from excessive wind speeds. “Wind stow” is an attitude of the tracked structure that presents the smallest surface area to the wind. Typically, a wind sensor will trigger a command for the system to assume the wind stow attitude.
There is delay associated with sensing an excessive wind speed, commanding the system to enter into a wind stow mode, and to physically assume the wind stow attitude. This delay may be acceptable when the wind is steady but increasing. However, there are several weather phenomena that can result in sudden, unpredictable increases in wind speed and direction. For example, a dust devil is a small, rapidly rotating column of wind resembling a small tornado that is made visible by the debris it picks up. A dust devil usually occurs in arid or semi-arid areas and is most likely to develop on clear, dry, hot afternoons in response to surface heating. Another weather phenomenon is known as a downdraft which is a sudden descent of cool or cold air to the ground usually with precipitation, and associated with a thunderstorm or shower. A downburst is a severe, localized downdraft from a thunderstorm or shower. An even more extreme weather phenomenon that can occur anywhere in the world, given the right conditions, is a tornado which is a violently rotating column of air. These sudden shifts in wind speed and/or wind direction can cause severe physical damage to celestial tracking systems and nearby property before such systems can be placed in a wind stow attitude.
Accordingly, what is needed is a positioning system for a large object that overcomes the problems of positional uncertainty found in prior art devices. Moreover, what is needed is a positioning system for a large object that works as an adjunct to, or in lieu of, a wind stow mechanism for protecting the object from sudden, unpredictable changes of wind speed and wind direction.